The following materials have been used to manufacture optical components, such as lenses included in optical devices such as optical pickups, as disclosed in Patent Document 1 or Patent Document 2: glass, plastics, and monocrystalline lithium niobate (LiNbO3).
Since glass and plastics have high light transmittance and can be readily processed into products having a desired shape, they are principally used to manufacture optical components such as lenses. On the other hand, since monocrystalline lithium niobate has high electro-optical properties and is birefringent, it is principally used to manufacture optical components such as optical waveguides. Small-sized and/or thin optical devices, such as optical pickups including such optical components, are demanded.
Since known types of glass and plastic have a refractive index of less than 1.9, optical components made of such materials and optical devices including such optical components have limitations in size and thickness. In particular, the known types of plastic have a disadvantage that incident light cannot be efficiently transmitted therethrough and gathered therewith because they have low humidity resistance and are birefringent.
On the other hand, monocrystalline lithium niobate has a large refractive index of, for example, 2.3. However, monocrystalline lithium niobate has a disadvantage that it is unsuitable for optical components such as lenses and has limited applications because it is birefringent.
Examples of a material which is not birefringent and which has satisfactory optical properties include translucent ceramics principally containing Ba(Mg, Ta)O3 or Ba(Zn, Ta)O3 perovskite. Such ceramics are disclosed in Patent Documents 3 and 4.
In the translucent ceramic disclosed in Patent Document 3, principally containing Ba(Mg,Ta)O3 perovskite, the optical properties, such as the refractive index and the Abbe number thereof, can be varied by partly replacing Mg and/or Ta with Sn and/or Zr, that is, a tetravalent element. An increase in the number of replaced elements increases the changes in the properties. However, it is difficult to greatly vary the refractive index and the Abbe number of the translucent ceramic because the upper limit of the number of replaced elements is small, 0.40. The refractive index thereof can be varied in the range of, for example, 2.071 to 2.082.
In a translucent ceramic, disclosed in Patent Document 4, principally containing Ba(Zn, Ta)O3 perovskite, Zn and/or Ta can be replaced with Zr; however, the upper limit of the number of replaced elements is small, 0.06. Therefore, it is difficult to greatly vary the refractive index and the Abbe number thereof. The refractive index thereof can be varied in the range of, for example, 2.128 to 2.132.
Thus, the use of the translucent ceramics to manufacture optical components leads to a decrease in the degree of freedom in designing optical devices.
In general, the linear transmittance of visible light through an optical component such as a lens is preferably independent from the wavelength. Therefore, there is a problem in that a decrease in wavelength reduces the linear transmittance of an optical component made of one of the translucent ceramics.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-127078 (all pages and FIG. 1)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-244865 (claim 6 and paragraph [0024])
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-75512 (all pages and figures)
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2004-75516 (all pages and figures)